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Post by Tom Goodrick on May 31, 2010 21:01:53 GMT -5
This is a good place to discuss the stall characteristics of the DHC-6 as modeled in FS9. This aircraft is of interest because it has special design features that give it the ability to make short flights with quick visits to small airports. I flew on one in 1979 in the Houston area from the large Bush International Airport to a small airport near the NASA base. It is not a pressurized aircraft so it stays low and goes quickly between airports. I had a good seat where I could watch the crew and see the view through the cockpit windshield. They flew steep approaches. The wing is small with high-lift flaps to give it a STOL (short takeoff and land) capability. When the flaps are stowed in the wing, it has a high wing loading for fast flight. Its fast cruise is 183 knots. I am guessing a typical landing speed is about 60 knots.
I must add that I don't have good documentation of the DHC-6 as I normally have when I do the FD for an aircraft. The period of the late 1960's to the mid 1970's when this aircraft was developed is not well covered in my library.
The reason we are discussing this is that Lou Ross has a bunch of flight time as pilot of a Twin Otter so he can fill in some details about its performance and flight characteristics. He expressed dissatisfaction with the FS models of this plane he has found. With his help we should be able to set it up for more realistic flight.
I have a Blue Grass DHC-6 that I got when we were all flying FS2002 and then I converted it to fly in FS2004. I did some stall work this morning and will describe what I found. But first we have to talk about weight because that plays a big part in what we experience in stalls. The max gross weight of mine is 12494 lbs when fully loaded with passengers, baggage and just enough fuel to reach the max gross weight. The CG was at 28.06% mac.
I figured it would be hard to find passengers to volunteer for a series of stalls so I did the stall tests with just the crew and 50% fuel. The gross weight was 9409 lbs and the CG was at 12.95%. (That CG position depends on my interpretation of Lou's description of the position of the two fuselage tanks.)
Normally, we see stall speeds for a clean wing and for a wing with full flaps in the literature. I have not even been able to find those figures. I just guessed at them when I set up the model.
I tested clean stall with twin power ON and OFF and then with only the left engine ON and then with only the right engine ON. Then I repeated the tests with full flaps.
Clean, both engine full power: 62 kIAS Clean, both engine idle: 71 KIAS Clean, Left engine full power: 70 KIAS, fell off to the right, Clean, right engine full power: 70 KIAS, straight ahead.
In all cases recovery was easy with a fair amount of forward stick and moderate throttle to get flying speed, then climb power.
Flaps, both engine full power: no stall. Hung at 52 KIAS. Flaps, both engine idle: 46 KIAS Flaps, left engine full power: descending slightly at 51 KIAS Flaps, right engine full power: Dropped at 46 KIAS and then recovered to hang at 51 KIAS.
Again, recovery was very easy using standard methods.
It is clear that there is plenty of power in these engines to cause a lightly loaded aircraft to hang on a combination of wing lift and thrust. To get a stall with power on I had to do a pullup from 90 KIAS with some over-rotation.
The amount set for max elevator trim has a lot to do with this. I had 19.5 degrees of max elevator trim. It would be easy to increas this max trim angle. For stall you set the max trim angle and then pull the stick back for max elevator deflection.
Note that FS uses and has always used a peculiar and unrealistic pitch control method that is close to the old way Piper and some others (Boeing) have done it. At least this makes it plausable to me. There is a trim adjustment in which the horizontal stabilizer is adjusted through an angle. Then the elevator is deflected through an angle. The effects are addative. You can get the same effect by adding trim and elevator deflection. In normal flight it gives you the realistic flying technique where you trim for speed in the clmb, cruise and descent phases and move the elevator with the stick only for transients like the takeoff rotation and the landing flare.
The method used for pitch control in many real aircraft are significantly more complex. Thus the detail effects the pilot must handle are different, especially in dynamic maneuvers. But there are still many things we can do to make the piloting job more realistic. However, to do that, we need the input of pilots experienced in the actual airplane.
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Ed Burke
Member
Healthy living is fine, but it's having fun that keeps us going!
Posts: 433
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Post by Ed Burke on May 31, 2010 22:39:35 GMT -5
Not much to do with stall characteristics of the Twotter but a few years ago I had a flight from Aeropelican YPEC near Newcastle, New South Wales to Sydney YSSY in one. A short trip but a memorable one due to the clear skies and a maximum of 5000'. The views were wonderful and unobstructed by hardware.
I was musing about how the approach into a major airport would be handled in a true STOL aircraft. So I watched carefully as we did a very conventional interception of the glide slope and aimed for the threshold of runway 34R. All was, apart from the speed, precisely what we would have been doing in a B737 until we flared for the landing. Then a tweak of power and we flew the entire 7000 feet of the runway at 6' agl until a gentle touch down and a burst of reversed props saw us enter the taxiway at a nice legal pace. Very, very neat indeed and we made surplus 99% of the blacktop.
Mentioned this to the pilot as were were leaving the cabin and he just grinned and said "Dead easy mate but they still charge us landing fees".
Ed
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Post by Tom Goodrick on Jun 1, 2010 9:02:55 GMT -5
I took a few minutes last night, intending to try some accelerated stalls. This time I loaded the aircraft to max gross by filling the aft fuel tank in the belly. (I probably have it too far aft.) My CG was at 32% mac.
I did all kinds of terrible things to the aircraft such as jerking it out of 90 degree banks. It just recovered easily. There were no nasty break-aways or other adverse dynamics. So after fooling around like that I found I was near Guntersville (8A1). That's a nice little 3200 ft strip on the edge of a lake. I set up an approach and expected a smooth approach and landing. I did slow it down a bit more than I usually would. I had those numbers from last night in mind. But with the higher weight, those stall speeds would increase by about 15%.
Pitch control on short final seemed a bit mushy and before I could recover, I was on the way to a dump in the lake! Way too late I gave it full power but ti still splashed into the water.
The stall has a bite after all if this model is any good.
How do airline pilots practice stalls without using an empty airplane that would have speeds a lot slower than when normally loaded?
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Post by louross on Jun 1, 2010 9:39:58 GMT -5
Okay, got it downloaded, will check it out this week and also make a few prelim comments, hopefully later to day. lr.
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Post by Tom Goodrick on Jun 1, 2010 13:42:30 GMT -5
I hope you have downloaded a freeware aircraft. We cannot have a public discussion about a commercial model on this Forum. That would violate the copyright.
I found some information about the DHC-6-300 on some websites this morning. I try to avoid web sites for info becaue I don't give them any credibility. Published books have been screend for accuracy to some degree.
On a NOAA government website they discuss the DHC-6 as they fly it for their various weather-related studies. They state the max takeoff weight is 12,500 lbs, the Empty weight is 8100 lbs. A private website (seabee.info/dhc6.htm states that the max landing weight is 12,300 lbs. and that the stall speed is 56 KIAS. That is important (if true) because the stall speed with flaps is generally specified for max landing weight while the clean stall speed is generally specifed for max takeoff weight. (slight difference in this case.) That stall speed and weight fixes the max total lift coefficient (basic plus flaps).
They also note that the wing span is 65 ft and the wing area is 420 sq ft. That gives an aspect ratio of 10.06 which is pretty high. But taht would be expected for this aircraft. They listed a wing loading of 29.8 sq ft, also expected. Max operating speed is given as 170 knots, presummed KIAS.
NOAA states they normally cruise for 5 to 6 hours at 130 knots of 110 knots in "survey mode" where they are taking data. At normal cruise they see 580 pph (total).
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Post by louross on Jun 1, 2010 16:59:05 GMT -5
Okay, let me start with this- I was rather impressed with the flying of the model you provided the link to.
When I flew them the MGTOW was 11,579lbs. I believe this was an arbitrary figure due to certification standards. Since then the FAA has instituted new standards which allowed commuter and regional airlines to fly heavy equipment without having to be certified as 121 carriers. Apparently the newly certified weight is 12,500lbs. (The DHC6 as originally built would definitely fly at weights heavier than the certification allowed.) Obviously the empty weight is going to vary with each individual a/c.
We flight planned at 180 TAS and fuel at 300 lbs/hr. Stall: maintain level flight and power back to idle, maintain altitude with strong back pressure, airplane stalls- continue hoding full back yoke- add power- the airplane recovers- no forward yoke required! This model did about the same. I'd call that pretty good.
Standard take-off with 10* flaps, props at 75%. Plenty of power. That's for standard airport ops. More prop and the noise can be unbearable. This model did fine.
Power lever-torque: this is the best I've seen in FS (for PT6A engine). Reducing power, the torque gauge may lag a bit more than in the real airplane, but again, this model is very good in that respect.
A couple of points: (1) the ITT gauge reads way too high. (2) cockpit modeling is,mmm, well, not at all real. PAD does have some cockpit models that actually look like a real Twin Otter. Unfortunately, the various controls don't work correctly. (3) BETA Range of the PT6A is not modeled. Sound pkg is apparently the default King Air. (4) Should not be any "aux tanks".
Also, did a STOL landing and the only thing was on the approach with full flaps and props full forward (STOL procedure) the model should be more nose down. lr.
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Post by Tom Goodrick on Jun 2, 2010 9:06:02 GMT -5
I did more stall testing, this time at a heavier weight after correcting the basic wing max lift. I calculated the value of max lift coefficent needed to fly at 56 KIAS. It was just a little (0.07) more than the 2.68 already there as a combination of basic wing and full flaps. I thought the other day that the clean stall was abit high so I added the .07 to the wing, not the flaps. It worked out well. At 11,893 lbs I saw a stall at 74 KIAS. It was just as you decribed. I was level and moved the stick back until it stalled as shown by a drop in altitude. Then I applied high power and it flew out of the stall. I did the stall with full flaps the same way with the same result only the speed was 56 KIAS.
I made some slight adjustments that may help the power respons and the nose attitude in a STOL approach. On the STOL approach did you men the nose did not appear low enough when viewed from the spot plane or from the cockpit? My fix applies to the external view.
To fix a problem with the internal view, look in the panel.cfg file in the panel folder to see if there is a line like this:
[Views] VIEW_FORWARD_DIR= 2.5, 0.0, 0.0
The 2.5 is the downward slant in degrees of your view over the panel. Change that number to suit your taste. This is the line for my panel. (You wouldn't like my panel - it's a modern digital glass panel.)
I'll send you the two files I modified: aircraft.cfg and DHC6W_22.air. I will rename them with extensions .ctg and .atg so you can protect the files you have. (give them names like .old to save them). To run, "save as" these files with extensions .cfg and .air.
(Any file ending in .cfg is just a text file that can be read and edited with Notepad.)
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Post by louross on Jun 2, 2010 9:40:31 GMT -5
In the real a/c you definitely notice more nose down than on a normal approach. In the sim, the STOL approach looked like a normal approach (cockpit view) and in spot view the a/c looked very slightly pitch down.
Any ides about the models with a real cockpit? If I install a medel with a real DHC6 cockpit, can I just replace the cfg files? lr.
PS: in training/flight checks the actual gross weight is not important. If you can fly and handle the plane at lighter weights you can fly/handle it at heavier weights. Pilots checked out on the 727 (-100) moved right over to the stretch (-200) version with no thought about it. A flight check in the -200 isn't required.
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Post by Tom Goodrick on Jun 4, 2010 8:49:47 GMT -5
There is one other thing about those files I sent. They load the plane full up with passengers and baggage. If you start with a flight situation that loads the tanks full, you are 2000 lbs over gross and climbing is very poor. You can edit the aircraft.cfg file to remove some passengers or baggage and you can change the fuel as needed. I fly with 40% in the front tank and zero in the back tank. I think I have the total fuel right but am not sure of the amount in each tank.
Could you please describe how you do a STOL landing? I am having trouble. I get on what seems to be a good approach angle and then it comes up short and a lot of power is needed to get to the runway. It might be useful to know which runways you used for STOL approaches. I am coming into a runway next to water. I get wet every once in a while.
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Post by Tom Goodrick on Jun 4, 2010 10:12:21 GMT -5
We still need to work on the stall characteristics because I am having too much trouble rounding out from a steep STOL descent. Lets look at the heart of the matter - the relation of the aircraft's inate pitching moment to its angle of attack (table 473 in the air flie). This figure shows a portion of that relation - the portion between zero and the stall angle and a little beyond. The dot at the angle of .32 radians (about 18 degrees) coincides with the angle at which the lift coefficient peaks before dropping off. (I checked on this in table 404 and made sure these points did coincide.) This table 473 gives the inate pitching moment the FD designer builds into the aircraft covering angles of attack from -180 to +180 degrees - every orientation the aircraft could possibly see, even if tumbling nose over tail. To fly the airplane the sim pilot sets a combination of elevator angle and elevator trim angle which, with the airplane at a particular angle of attack, raises a point from Cm=0 straight up to this line where it passes over that angle of attack. When the point reaches the line there is no resultant moment. If the point goes above this line there is a nose up moment causing angle of attack to increase. If it's below the line, the nose and angle of attack go down. home.hiwaay.net/~goodrick/DHC-6 Stall.jpg[/img] This is for the basic wing with the tail set at zero trim and zero elevator deflection. Flaps add a small moment raising (or lowering) this line (but their effect is never as much as the tail moment). I have already modified this chart from what it was in the files I previously sent you. In those files it goes straight to the right at the stall point instead of rising steeply. When we run through a practice stall we add elevator raising a point slightly above the line to cause an increase in the angle of attack. When we pass beyond the stall point, the loss of lift cause the aircraft to drop a little with the tail tneding to drop the nose a little (the pitch moment of inertia determines how fast this drop occurs). In the case I have shown here, the moment need to hold the angle of attack rises steeply so the chances are good that the total effect is to drop the nose and hold it at the stall point where there still is good lift. Many aircraft have a reduced slope or even a flat slope as I had the original chart. This can cause the aircraft to proceed into a deeper stall making recovery a little more difficult. Indeed some larger jet aircraft can go into such a deep stall recovery is nearly impossible, I suspect this is the case with the Dash 8, especially with some ice on the wing. In a normal stall, the wing loses lift rapidlt and this contributes to self recovery because the tail is still flying and causes a strong nose-down moment after the loss of lift. This would be the case with a steep moment curve after the stall as shown here. So the question is, Lou, if you could not use high thrust from the props to climb out of your gentle stall, what would the aircraft do? Would you have to push the yoke forward to unstall the wing or would it self-recover indicating the curve is like this one? I just flew the plane with this moment curve and had another firm landing (-1300 fpm). If I had used power I could have rounded out better.
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Post by louross on Jun 4, 2010 15:34:57 GMT -5
You're going to have to bear with me here for a minute. I have the 2 files- aircraft.cTG and DHC6W_22, which has the blue MS symbol for the notebook. Now to confirm, I need to put the current a/c cfg into a folder in order to save it, and place these 2 files in the main a/c folder of the DHC6. Correct? -OR- I need to make a copy of the original a/c cfg and open it in notebook then replace that text with the text of aircraft .cTG. Also remove the letters "DG" form the panel line. Are both of these ways the same thing? Should I eliminate the blue DHC_22 notebook symbol and re-download the original DHC6_22 icon and use it instead? lr.
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Post by louross on Jun 4, 2010 15:55:31 GMT -5
Two responses since my post of Jn 2.
1) Correct, full pax and loaded with cargo will limit your fuel load. 2) As I remember, power-off stall with the yoke held full back- add power and the a/c recovers. Otherwise, my guess is you must move the yoke forward then recover. Usually in power-off stall training, power was added as the yoke pressure was released to allow some elevator movement. The a/c did stall straight away, I don't remember much in the way of yaw. It was a very stable airplane.
Also, this just came to mind, STOL Take-Off: Props Full Forward Hold Brakes Yoke full back (into your stomach/chest) Add Full Power / release Brakes A/C rolls and immediately/quickly as it lifts push yoke forward to slightly nose down and it continues lifting, pretty much level attitude. lr.
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Post by Tom Goodrick on Jun 4, 2010 22:54:26 GMT -5
"I have the 2 files- aircraft.cTG and DHC6W_22, which has the blue MS symbol for the notebook."
That second file is actually DHC6W_22.aTG . Disregard any icons.
You don't have to do any editing of the files, just of the names of the files.
It is best if you make two sub folders (maybe call them "OLD" and "NEW") and copy you existing aircraft.cfg file and youe existing DHC6W_22.air file into one to save them. Then copy these two new files into the other folder to save them. Then delete your aircraft.cfg file in the DHC-6 folder and delete the DHC6W_22.air file in the DHC-6 folder. Copy in the two new files and change their names: aircraft.cTG becomes aircraft.cfg and DHC6W_22.aTG becomes DHC6W_22.air . After this you will be flying with the new flight dynamics.
Let me know what you think about the performance after running the plane through some exericises with these new files. I already have some improvements in mind but I will make more adjustments to cover any problems you find. Then I will send you another pair of files and you can repeat this exercise.
The reason we replace whole files is that you would have a tough time finding where I made the changes in each file. It is best just to swap files.
The airplane folder must have one file named aircraft.cfg and another file with a name related to the airplane that ends in .air . Without these files the airplane will not load or fly in FS.
Only the aircraft.cfg file can be easily edited using Notepad. The file ending in .air can only be edited using a special air file editor. Today most people do not need an air file editor because most of the changes you might want to make can be made with just the aircraft.cfg file.
But one change that must be made using an air file editor is this matter of the stall characteristics as mentioned above with the Cm graph. It is tricky and can easily make the aircraft unflyable if done wrong.
I know this stuff is strange and a little hard to learn, but you will be able to find improvements in your flying if you learn to swap out these files.
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Post by flaminghotsauce on Jun 5, 2010 5:09:14 GMT -5
I was trying this aircraft out as well, but I have a problem with my prop lever in FS9. I cannot adjust prop 1. I may have to go the FSX route on this one. Neither can I balance the throttles and mixtures. Very odd. I tried the throttle quadrants with the Baron and all worked correctly except prop 1, so I'm having some type of difficulty with this aircraft outside the FS9 prop 1 difficulty.
Dang it. I never have found a twin otter that worked for me. I guess I'll have to search out payware.
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Post by louross on Jun 5, 2010 11:35:38 GMT -5
Well, hey, by golly, flaminghotsauce- that reminds me! The center power lever panel for the twin otter: prop levers are brought to feather for engine shut-down; they remain feathered for engine start. Now a default probelm with FS: the right 2 levers in the twin otter are fuel levers, as mixture levers are different, twin otter being a turboprop- not a piston. In MSFS they are set at full forward, and as you retard the levers you'll notice that the torque and N1 decrease. The fuel levers should either be at the cut-off position, or, at the lowest, most idle setting you can get and still keep the engines running. (This is the case with all PT6A engines, which are incorrectly set up in FS. Especially the KingAir series. In those a/c in real, the condition lever- as called in those a/c- is either at cut-off, or, "low idle". Never heard of anyone using "high idle" in those types a/c- because, it's not necessary. There are times you might use it for a second or two during engine start under certain circumstances). AND ALSO, I forgot to say to set the flaps before you set the brakes for the STOL takeoff! For STOL, set them at,,,,, I have no idea!!! Completely forgot!!! Can't synch the prop levers? Haven't heard that one. Some kind of saved cfg? Probably no big deal- normal flying is always at 75-80% prop. Feather the props, move each one out of feather, then forward a bit, or maybe just a titch more, and leave them there for T.O, climb, cruse, approach and landing. That's common procedure. If you are making a STOL landing: props are at 75-80% and lined up with the runway; descending on the glide slope (ILS or visual) at 160 knots; about 1.5 or 2.0 miles from the end of the runway, power back to idle; push props full forward (easy to do individually at idle); begin slowing and start flaps down at 120 reducing speed to 70; (add power if necessary on short final) power idle; touchdown; full reverse thrust (F2). You can land on 32R at KORD and turn left at the 27R parallel taxiway- without brakes). Idea: hold 160 just up to the 4-lane highway about a mile ( ) or there abouts before the runway end (FS9), IDLE, PROPS FORWARD, slowing to 120 start flaps full down, ADD Power about 1/2-3/4 mile out, down to runway, power idle, on the ground, reversing. lr.
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